System for electrochemical detection of chlorate explosives

ABSTRACT

A method of detecting chlorate in soil includes contacting soil wetted with a solvent containing an electrically conductive salt with an electrode comprising layers of vanadium-substituted phosphomolybdate alternating with layers of para-rosaniline, and performing voltammetry with the electrode, wherein a catalytic reduction current indicates a likelihood of the presence or absence of chlorate in the soil. A system includes a potentiostat operably connected to the electrode and in communication with hardware and software sufficient to produce an output indicating a chlorate level in soil.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit as a division of U.S. patentapplication Ser. No. 15/234,600 filed Aug. 16, 2016. Moreover, thisapplication is related to commonly-owned US Patent ApplicationPublication 2016/0061775.

BACKGROUND

The manufacture and use of improvised explosive devices (IEDs) presentsa serious hazard to military and civilian personnel in conflict zonesthroughout the world. Consequently, it is critically important to beable to identify and dismantle manufacturing sites for these devicesbefore they can be fabricated and deployed. A key element in thisstrategy is the detection of the presence of residual explosivecomponents at or near the sites, providing confirmation of illicitactivity and allowing authorities to coordinate efforts to identify anddetain persons associated with IED manufacture. As current detectionmethods for readily available nitrogen-based explosives continue toevolve in terms of sensitivity and selectivity,¹⁻⁵ however, enemycombatants are increasingly turning to alternate explosives based onchlorate or peroxides, for which fewer means of reliable detection inthe field are available.

In particular, chlorate detection is complicated by a complexpH-dependent redox chemistry that facilitates (inter)conversion ofchlorate to other chlorine-containing species such as hypochlorite,chlorite, chlorine dioxide, chlorine, and chloride.⁶⁻⁸ Ion exchangechromatography provides a means for rapid separation of suchCl-containing species and, coupled with sensitive mass spectrometricdetection, permits quantitative determination of each species in alaboratory setting.^(9,10) However, carrying out such an analysis in thefield, where rapid determinations must be made under often adverseconditions, presents serious logistical issues, especially in terms ofsafety, weight, and power requirements.

Simpler spectrophotometric methods for chlorate detection are, intheory, amenable for field use. These methods are generally based onbleaching of the color of a dye species^(6,11-14) or the catalyzedgeneration of colored triiodide anion¹⁵⁻¹⁸ in the presence of chlorate.The color change may be visible to the naked eye or with the aid of verysimple instrumentation, permitting development of a lightweight systemthat requires little or no power to operate. However, specificity forchlorate detection generally remains an issue, since various otherchlorine species such as hypochlorite, chlorite, chlorine, and chlorinedioxide are also strong oxidants capable of interfering and rendering afalse positive signal.

Electrochemical methods that exploit the redox behavior of the chloratespecies provide a convenient alternative to spectrophotometric detectionmethods. While electrochemical methods do require a power source, recentadvances in electronics miniaturization and design can now providelightweight, low-cost, rugged, low power potentiostats¹⁹ that limit theimpact of this issue. Because a simpler “yes or no” determination,rather than a quantitative chlorate analysis, may be sufficient foron-site testing in the field, any sensitivity issues related to the useof these simpler instruments are less of a concern. There are, however,several other issues associated with electrochemical detection ofchlorate that must be addressed, especially for field work.

First among these is electrode type. Previous systems for analysis ofchlorate were based primarily upon polarographic and related techniquesusing Hg electrodes,²⁰⁻²⁵ which are not ideal for field use. During the1990s, however, Gao and coworkers²⁶⁻³⁰ extended earlier work by Unoura³¹and others^(32,33) demonstrating catalytic electroreduction of chlorateby polyoxometalates and related transition metal compounds in solutionat Pt and glassy carbon electrodes by developing electrocatalytic carbonpaste electrodes impregnated with carboxylate ligand species andpolyoxometalates as chlorate sensors. More recently, Jakmunee andcoworkers³⁴ demonstrated amperometric detection of chlorate using atriiodide based scheme with stopped-flow injection, which wassubsequently used for successful detection of chlorate in soilsamples.³⁵

A second issue is the potential interference due to electrochemicalsignatures of other chlorine-containing species, such as hypochlorite,chlorite, and chlorine dioxide, which may be present with chlorate orgenerated from it during the course of sampling and analysis.Significant efforts and progress have been made to address this concern.For example, rather than detect chlorate directly, Wen and coworkers³⁶utilize it to selectively oxidize chalcopyrite (i.e., CuFeS₂) andelectrochemically detect the Cu(II) and Fe(III) released. Similarstrategies have been demonstrated using sphalerite (i.e., ZnS₂,producing Zn(II))³⁷ and galena (i.e., PbS, producing Pb(II))³⁸ as themetal ion sources. Elimination of Cl-containing interferences can alsobe accomplished via their preferential removal from a sample by reactionwith N₂H₅ ⁺/OsO₄,³⁹ BH₄ ⁻,⁴⁰ or Fe(II)⁶ and/or careful adjustment of thereaction conditions^(41,42) prior to initiating the electrochemicalanalysis of chlorate. Finally, recent work indicates that selectivesurface modification of the electrode with rare earth coatings^(43,44)can hinder the reduction of certain Cl-containing species, such ashypochlorite, in the presence of chlorate.

Despite these advancements, the electrochemical analysis of chlorateunder ambient conditions in the field remains hindered by the presenceof oxygen, whose reduction (E_(pc)>−0.3 V. vs. Ag/AgCl) is sufficientlyclose to that of chlorate (E_(pc)≅−0.4 V. vs. Ag/AgCl) to interfere withthe analysis. Although the pH dependence of the oxygen reductionpotential can be exploited to lessen this effect, it cannot be entirelyremoved. In similar fashion, any attempt to deconvolute measured currentdata to account for the oxygen contribution requires simultaneousmeasurement of the oxygen level and introduces additional complexity anderror sources into the analysis. Removal of oxygen from the sample bypurging with inert gas can certainly solve the problem, but at theexpense of longer analysis times and added inert gas container weight,both of which are problematic for field use.

G. Cao et al.⁴⁵ described a technique wherein sulfur-polyoxometalate(POM) is mixed with methylene blue dye into a carbon paste, which iscast as an electrode. In the presence of 1M H₂SO₄ (strongly acidicconditions), this electrode could detect chlorate, however the electrodealso exhibited significant instability. It is not apparent that thiselectrode could operate under, non-acidified conditions nor does notseem as if the electrode would have usefully long life in the field, andit must regularly by “renewed” by squeezing out fresh carbon pastecontaining the POM.

A need exists for an electrode that senses chlorate directly underambient conditions from real world samples, such as soil, that may alsobe contaminated with traces of other electroactive species, such ashumates, metal ions, and nitrogen-based explosives, among others.

BRIEF SUMMARY

One embodiment is a method of detecting chlorate in soil includescontacting soil wetted with a solvent containing an electricallyconductive salt with an electrode comprising layers ofvanadium-substituted phosphomolybdate alternating with layers ofpara-rosaniline, and performing voltammetry with the electrode, whereina catalytic reduction current indicates a likelihood of the presence orabsence of chlorate in the soil. This can be accomplished withoutexcluding or removing oxygen from the testing environment andfurthermore without the use of strong acid.

Another embodiment is a system for conducting the method, including apotentiostat operably connected to an electrode comprising layers ofvanadium-substituted phosphomolybdate alternating with layers ofpara-rosaniline, and computer hardware and software in communicationwith the potentiostat and configured to produce an output indicating achlorate level in soil in contact with the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of layer by layer (LbL) alternateelectrostatic deposition of the Keggin-type polyoxometalate anion,[PMo₁₁VO₄₀]⁵⁻ (PMo₁₁V), and the cationic dye, p-rosaniline acetate (PR)on indium tin oxide (ITO). FIG. 1B shows absorbance spectra as afunction of number of deposited PVMo₁₁/PR bilayers. Spectra in order ofincreasing absorbance at 535 nm are shown for 1, 2, 3, 4, 5, 6, 8, 10,12 14, 16, 18, and 20 bilayers. FIG. 1C shows para-rosaniline absorbanceat 535 nm vs. number of PVMo₁₁/PR bilayers deposited. Two linear fits ofthe data are shown. The data from n=0−6 obey the equation:A=0.0053n+0.002, r²=0.9907. The data from n=6−20 obey the equation:A=0.0078n−0.0164, r²=0.9973.

FIG. 2A shows cyclic voltammograms of PVMo₁₁/PR bilayers (n=6) on ITO asa function scan rate. Cyclic voltammograms of PVMo₁₁/PR bilayers (n=6)on ITO as a function scan rate. FIG. 2B shows cyclic voltammograms ofITO and PVMo₁₁/PR bilayers (n=5) on ITO comparing different films agedat different times. Scan rate 50 mV·s⁻¹. Electrolyte=100 mM sodiumchloride adjusted to pH 2.86 with HCl. Yellow=1 week; Gray=5 weeks;Blue=8 weeks (film 1); Orange=8 weeks (film 2). Current measurements forthe two films aged 8 weeks are identical within experimental error tothe current obtained for the film aged 5 weeks, indicating that anychanges in film structure are complete by 8 weeks and the films havereached equilibrium by that time. Identical currents within experimentalerror observed for the two 8 week old films demonstrate and confirm filmdeposition reproducibly.

FIG. 3 shows cyclic voltammograms of ITO and PVMo₁₁/PR bilayers (n=6) onITO. Dark blue=bare ITO; Dark green=bare ITO+0.015 mg·mL⁻¹ of TNT;Yellow=PVMo₁₁/PR bilayers (n=6) on ITO in aerated solution; Lightblue=PVMo₁₁/PR bilayers (n=6) on ITO in solution purged with Ar; Lightgreen=PVMo₁₁/PR bilayers (n=6) on ITO+0.015 mg·mL⁻¹ of TNT. Scan rate 50mV·s⁻¹. Electrolyte=100 mM sodium acetate at pH 2.5

FIG. 4A shows cyclic voltammograms of PVMo₁₁/PR bilayers on ITO withincreasing [KClO₃] concentration. FIG. 4B. shows current (Δi) vs.[KClO₃], μM (at −0.4 V. vs. Ag/AgCl) in aerated pH 2.5, 100 mM sodiumacetate buffer. Scan rate=50 mV·s⁻¹.

FIG. 5 is a normal probability plot of Effects, namely a plot of currentEffects from Table 11 vs. probability of random occurrence, P_(j).Effects comprising the straight line portion of the plot are ascribed torandom error. The Effects deviating from the line make statisticallysignificant contributions to the observed current. Letters identify thefactor or factor interactions corresponding to each point.

FIG. 6 is a normal probability plot of current residuals, namely a plotof current residuals (ΔR_(i)) from Table 16 vs. probability of randomoccurrence, P_(j).

DETAILED DESCRIPTION Definitions

Before describing the present invention in detail, it is to beunderstood that the terminology used in the specification is for thepurpose of describing particular embodiments, and is not necessarilyintended to be limiting. Although many methods, structures and materialssimilar, modified, or equivalent to those described herein can be usedin the practice of the present invention without undue experimentation,the preferred methods, structures and materials are described herein. Indescribing and claiming the present invention, the following terminologywill be used in accordance with the definitions set out below.

As used in this specification and the appended claims, the singularforms “a”, “an,” and “the” do not preclude plural referents, unless thecontent clearly dictates otherwise.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items.

As used herein, the term “about” when used in conjunction with a statednumerical value or range denotes somewhat more or somewhat less than thestated value or range, to within a range of ±10% of that stated.

Reference herein to the “likelihood of the presence or absence ofchlorate” includes, unless the content clearly dictates otherwise,either or both a numerical representation of an expected chlorateconcentration (in the desired units, for example molarity) or moresimply a binary “yes/no” output indicating the likely presence orabsence of chlorate

Overview

Described herein is a technique for the detection of chlorates,particularly chlorate explosives, that is operable in the presence ofoxygen and not influenced by residual contaminants such astrinitrotoluene (TNT) or nitrates. The electrode exhibited goodstability and resistance to degradation, with electrodes usable for aminimum of several months. The associated apparatus is light weight andprovides a signal rapidly, on the order of two to five minutes.

A composite electrode operates to electrochemically detect the presenceof chlorate explosive extracted from soil and other solid samples insolvent (e.g., water). The electrode comprises alternating layers of avanadium-substituted phosphomolybdate polyoxometalate species and thedye, para-rosaniline. The invention allows detection of chlorate in soilsamples obtained either directly by personnel or remotely via UAV todetermine whether an area contains improvised explosive devices (IEDs)or a building or other structure is being used as a manufacturing sitefor IEDs. Chlorate is detected by its catalyzed electroreduction tochloride in and on the composite electrode.

The present inventors tested the ability of vanadium-substitutedphosphomolybdate polyoxometalate/para-rosaniline electrodes⁴⁶ toefficiently reduce chlorate. In particular, the electrodes werecomposite, porous electrodes prepared via layer by layer (LbL) alternateelectrostatic deposition of the Keggin-type polyoxometalate anion,[PMo₁₁VO₄₀]⁵⁻(PMo₁₁V), and the cationic dye, p-rosaniline acetate (PR).Electrocatalytic waves for the reduction of chlorate were observed usingthese electrodes under ambient conditions with no interference fromoxygen. Described herein the preparation and characterization of anelectrode system with respect to the factors electrode film age (A),solution acidity/pH (H), cyclic voltammetry (CV) scan speed (S),chlorate concentration (C), and number of PMo₁₁V/PR bilayers (L) presentin the electrode film. Electrode performance is optimized using aTaguchi L16 array and a two-level full factorial design provides a modelpredicting current for the detection of chlorate as a function of theseparameters.

The examples use para-rosaniline/vanadium-substituted phosphomolybdatecomposite films in which the outermost film layer is para-rosaniline.Films in which the outermost layer is the vanadium-substitutedphosphomolybdate should also have catalytic activity for chlorateelectroreduction and detection, and thus are expected to operatesimilarly.

EXAMPLES

Materials—Deionized water of 18.2 MΩ·cm⁻¹ resistivity obtained from aMilli-Q Advantage deionized water system was use to prepare allsolutions and for all experiments. Filtered N₂ gas from liquid N₂boil-off was used for drying samples during film deposition. Allchemicals were used as received except where otherwise noted. Branchedpolyethylenimine (PEI; 50% wt. solution in water; M_(n)=60,000;M_(w)=750,000; [9002-98-6]), para-rosaniline acetate (PR; 90% dye;347.41 g·mole⁻¹; [6035-94-5]; ε_(540 nm)=6.19×10⁴ L·mole⁻¹·cm⁻¹;Caution—cancer suspect agent), vanadium (IV) oxide sulfate tetrahydrate(VOSO₄.4H₂O; 97%; 235.04 g·mole⁻¹; [123334-20-3]), sodium dihydrogenphosphate (NaH₂PO₄; ≥99.99%; 119.98 g·mole⁻¹; [7558-80-7]), sodiumacetate (NaOAc; ≥99.0%; 82.03 g·mole⁻¹; [127-09-3]), potassium chlorate(KClO₃; ≥99.0%; 122.55 g·mole⁻¹; [3811-04-9]; Caution—strong oxidizer),glacial acetic acid (HOAc; ≥99.7%; 60.05 g·mole⁻¹; ρ=1.049 g·mL⁻¹;[64-19-7]), methanol (CH₃OH; ≥99.0%; 32.04 g·mole⁻¹; [67-56-1]), andsodium chloride (NaCl; ≥99.99%; 58.44 g·mole⁻¹; [7647-14-5]) were allfrom Sigma-Aldrich Chemicals.N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (EDA; 95% technicalgrade; 226.36 g·mole⁻¹; ρ=1.019 g·mL⁻¹; [1760-24-3]) from Gelest Inc.was vacuum distilled (140° C., 15 mm Hg) immediately prior to use.Sodium molybdate dihydrate (Na₂MoO₄.2H₂O; 99%; 241.95 g·mole⁻¹;[10102-40-6]) was obtained from Strem Chemicals. Concentratedhydrochloric acid (HCl; 36.46 g·mole⁻¹; [7647-01-0]) and sulfuric acid(H₂SO₄; 98.18 g·mole⁻¹; [7664-93-9]) were both ACS Reagent Grade fromFisher Scientific. Quartz slides (50 mm×25 mm×1 mm) were purchased fromDell Optics, Orange, N.J. and indium tin oxide (ITO) slides (part no.CB-501N-S111), each bearing an ITO coating (R_(s)=5-15 Ω) on one side ofa piece of Corning 1737F aluminosilicate glass (75 mm×25 mm×1.1 mm),were from Delta Technologies Limited, Stillwater, Minn.

Na₄H[(VMo₁₁)O₄₀] (i.e., PMo₁₁V; 1839.21 g·mole⁻¹) was prepared atreduced scale with some modification of the literature method⁴⁷ asfollows: A solution of 0.44 g VOSO₄.4H₂O in 9 mL 0.10 M HCl (aq)solution was freshly prepared. A second solution containing 0.054 gNaH₂PO₄ and 1.21 g Na₂MoO₄.2H₂O in 20 mL water was then prepared. TheVOSO₄ solution was then added to the well stirred NaH₂PO₄/Na₂MoO₄.2H₂Osolution. The stirred blue-black solution (characteristic of the V^(IV)form of the product)⁴⁷ formed was titrated with 3.0 M HCl (aq) dropwiseto pH 3.5 using a pH meter. After stirring at room temperature for 30min, the blue-black solution was quickly frozen at −20° C. for 90 min ina freezer and transferred to a freeze dryer. After freeze drying 4 daysthe water had been removed, leaving 1.53 g of a gray-black solid thatdissolves readily in water to form a dark blue solution that slowly(hours) becomes orange-brown in color. The gray-black solid was usedimmediately for film depositions, but can also be stored in the −20° C.freezer with no apparent change in color by eye for at least 4 months.

Solutions—Stock PEI (˜5 mg/mL) solution was freshly prepared just priorto use by weighing 1.0 g 50% wt. PEI (aq) solution into a tared 125 mLEhrlenmeyer flask, adding sufficient water to bring the total solutionweight to 100 g, and carefully stirring the mixture until the PEI hadcompletely dissolved. A 0.10 M HOAc (aq) solution was prepared bypipetting 2.90 mL HOAc into a 500 mL volumetric flask containing 100 mLwater and diluting to the mark with water. A 0.10 M NaOAc (aq) solutionwas prepared by quickly weighing 0.820 g anhydrous NaOAc into a 100 mLvolumetric flask, adding ˜30 mL water to dissolve the solid, anddiluting to the mark with water. Stock pH ˜4/0.10 M acetate buffer wasprepared by pipetting 90.00 mL of the 0.10 M NaOAc (aq) solution into a500 mL volumetric flask and diluting to the mark using the 0.10 M HOAc(aq) solution, yielding a solution having measured pH=3.99±0.02. Stock˜0.14 mM PR solution was prepared by weighing 5.3 mg of PR into a 100 mLvolumetric flask and diluting to the mark with stock pH ˜4/0.10 Macetate buffer. Stock PMo₁₁V (2 mg/mL≅1.09 mM) solution was freshlyprepared immediately prior to use by dissolving ˜22 mg ofNa₄H[(VMo₁₁)O₄₀] solid in 11 mL of the pH 4/0.10 M acetate buffer.

Chlorate-containing solutions for initial experiments were prepared bydissolving appropriate quantities of KClO₃ in a 100 mM sodium acetatesolution whose pH had been adjusted to pH ˜2.5 using hydrochloric acid.Chlorate-containing solutions having μ=0.10 M total ionic strength forelectrochemical studies related to the Taguchi and two-level fullfactorial statistically designed experiments were prepared from thefollowing stock solutions: Solution A (0.095 M NaCl (aq)) was preparedby dissolving 1.388 g NaCl in water in a 250 mL volumetric flask anddiluting to the mark with water. Solution B (0.100 M NaCl (aq)) wasprepared by dissolving 5.844 g NaCl in water in a 1 L volumetric flaskand diluting to the mark with water. Solution C (0.0625 M HCl(aq)/0.0375 M NaCl (aq)) was prepared by dissolving 1.096 g NaCl in ˜100mL water in a 500 mL volumetric flask, adding 2.60 mL concentrated HClby pipet, and diluting to the mark with water. These stock solutionswere prepared and stored until needed for experiments. Solution D (0.005M KClO₃ (KClO₃ (aq)/0.095 M NaCl (aq)) was freshly prepared each day byweighing 61.3 mg KClO₃ into a 100 mL volumetric flask, dissolving thesolid in ˜30 mL Solution A, and diluting to the mark with Solution A.Chlorate-containing solutions with specific pH and [KClO₃] values forelectrochemical studies were prepared by mixing Solutions B, C, and D inthe appropriate ratios. Corresponding blank solutions for thedetermination of background currents were prepared by replacing SolutionD in each formulation by an equivalent volume of Solution B.

Instruments and Measurements—Solution pH values were measured using aCorning Pinnacle 530 pH meter equipped with an AccuTupH+ pH electrode(cat. no. 13-620-185). UV-visible spectra were measured using a doublebeam Varian Cary 5000 spectrophotometer. Spectra of PMo₁₁V/PR films werecorrected for baseline variations using EDA-coated quartz referenceslides, prepared as described previously,⁴⁸ as blanks. Solution spectrawere recorded vs. solvent blanks in matched 1.00 cm or 0.10 cmpathlength quartz cuvettes. A VirTis benchtop K freeze dryer was used toisolate solid Na₄H[(VMo₁₁)O₄₀] following its preparation (vide supra).All cyclic voltammetry (CV) measurements were performed using athree-electrode configuration with a model 440 electrochemicalworkstation (CH Instruments, Austin, Tex.) interfaced to a GatewayE-1200 personal computer for data acquisition and processing. The ITOworking electrode coated by the PMo₁₁V/PR film, Pt wire counterelectrode, and Ag/AgCl reference electrode (Cypress Inc.) were used in astandard Teflon electrochemical quartz crystal microbalance (EQCM) cell(CH instruments, Austin, Tex., cat. no. CH1127) that was securelymounted and reproducibly positioned using a chuck on a Mellis-Griotoptical bench as described in detail previously.⁴⁹ For the Taguchi andtwo-level full factorial design experiments, the Teflon cell wasreplaced by a rectangular Press-to-Seal Silicone Isolator gasket (GraceBio-Labs, Inc., Bend, Oreg.; catalog no. 664116; 50 mm×25 mm×1 mm). Thegasket contained an array of 10 isolated chamber holes 7 mm×7 mm×1 mmeach) and formed a water-tight seal when clamped on the ITO electrodesurface,⁵⁰ permitting up to 10 separate and independent currentmeasurements per ITO electrode to be made. Each ITO/gasket well was usedfor an individual measurement of the chlorate blank, followed by theappropriate chlorate-containing solution to eliminate electrode crosscontamination possibilities. Before the chlorate measurement, themultilayers with the corresponding pH solution were equilibrated by 4successive CV sweeps from 0.8 V to −0.45 V vs. Ag/AgCl. Measuredcurrents were corrected for background capacitive contributions at 0.7 Vvs. Ag/AgCl where only non-faradaic processes were observed in thevoltammograms. The net currents were averaged from three corrected CVscans using E_(pc)=−0.4 V vs. Ag/AgCl for the electrocatalytic chloratereduction wave.

LbL Film Depositions—Quartz slides were cleaned by successive 30 minimmersions in 1:1 v/v HCl/CH₃OH and concentrated H₂SO₄ with copiouswater rinsing after each treatment per the literature procedure.⁴⁸ Forinitial experiments, the ITO slides were cleaned by rinsing with waterand soaking for 20 min in H₂SO₄, followed by copious rinsing with water.Because H₂SO₄ slowly removes ITO from the glass surface during thecleaning process, ITO substrates used for the Taguchi and two-level fullfactorial design experiments for optimization and modeling of electrodeperformance were cleaned in H₂SO₄ for only 10 min to maximize thecurrent response and current response differences among the conditionstested. Cleaned quartz and ITO substrates were immediately coated withPEI film by immersion in the stock PEI solution for ˜1 h, followed bytriple rinsing with water and drying in a stream of filtered N₂ gas.PEI-coated substrates were stored in Parafilm™-sealed capped Coplin jarsuntil needed for experiments. The PEI-coated quartz and ITO substrateswere used within 1 week for film deposition.

For LbL deposition of the PMo₁₁V/PR films, the PEI-coated quartz and ITOsubstrates were loaded onto a glass carousel, which was then immersed inthe stock 2 mg/mL PMo₁₁V solution (i.e., ˜1.09 mM) for 10 min. Thesubstrates were then triple rinsed in water and dried in the stream offiltered N₂ gas. Afterwards substrates were immersed in the stock 0.14mM PR solution for 10 min, triple rinsed in water, and dried in thefiltered N₂ gas stream. This sequence was repeated until films bearingthe desired number of PMo₁₁V/PR bilayers, “n”, of structureSubstrate/PEI/(PMo₁₁V/PR)_(n) were deposited. FIG. 1A is a schematicillustration of this process. All films having n≤20 were depositedwithout interruption during a single day of work. The samples werestored in Parafilm™-sealed capped Coplin jars for up to 8 weeks untilneeded for experiments. UV-visible absorbance spectra of the films wereperiodically recorded as a function of the number of PMo₁₁V/PR bilayersdeposited.

Statistically Designed Experiments—The Taguchi design consisted of anL16 array that examined the importance of the L, H, C, S, and Avariables in 16 total experiments, with each variable assessed at 4levels to determine the conditions for optimum (i.e., maximum current)electrode response. A two-level full factorial design was subsequentlyemployed for the analysis of the effects of the L, H, C, and S variablesfor electrodes aged A=8 weeks to provide a quantitative model relatingcurrent response to the levels of these variables. For each design,experiments were performed in random order to minimize the effects ofcumulative error. While the two-level factorial design experiments werecompleted in ˜6 h during a single day, the Taguchi design required 8weeks in order to assess the effect of the film age variable, A. Duringthis time the laboratory temperature was maintained at 22±1° C. withrelative humidity at 45±5% and the film-coated electrodes were sealed inCoplin jars to control environmental effects that could introduceadditional error into the measurements. Current measurements obtainedfor the Taguchi design were analyzed by the mean statistical analysisapproach⁵¹ to determine the effects of each variable. Correspondingstatistically significant effects and residuals for the two-level fullfactorial design were determined using the Yates' Algorithm and ReverseYates' Algorithm, respectively, leading directly to a predictive modelof the dependence of the electrode current on the statisticallysignificant L, H, C, and S variables and interactions.⁵²

Results—The preparation of bilayer films from PMo₁₁V and PR via the LbLapproach on quartz slides and glassy carbon electrodes (GCE) coated byPEI was originally described by Fernandes, et. al.⁴⁶ They demonstratedmonotonic (i.e., nearly linear) film growth as a function of number ofbilayers, with overnight interruptions of the deposition process leadingto variations in the subsequent growth rate consistent withreorganization of internal structure during film aging. Thicknesses aslarge as ˜150 nm were observed for films containing 20 bilayers, withmeasured roughnesses of ˜64 nm. Film morphology characteristic of aglobular, rather than stratified structure, was also noted consistentwith roughness results and electrochemical measurements indicatingpermeability of the films to ferrocyanide redox probe anions. Reversibleone-electron waves in the film CV due to V^(V/IV) at E_(pc)=0.367 V andMo^(VI/V) at 0.098 V, −0.122 V, and −0.266 V vs. Ag/AgCl furtherconfirmed redox activity and accessibility of solution species to thePMo₁₁V component. Film growth was rather insensitive to [PMo₁₁V] in the1.0 mM≤[PMo₁₁V]≤5.0 mM range for n<8 bilayers, though somewhat thinnerfilms were observed using the higher [PMo₁₁V] in this range when filmshaving n 8 bilayers were examined. However, deposition times morestrongly influenced film growth, with 10 min treatment times using thePMo₁₁V and PR solutions providing thicker films than 5 min or 20 mintreatments.

Given these favorable characteristics, PMo₁₁V/PR films were fabricatedon PEI-coated quartz and ITO slides for chlorate electrocatalysisstudies using 2 mg/mL (i.e., 1.09 mM) PMo₁₁V and ˜0.14 mM PR solutionsin pH ˜4 0.10 M acetate buffer with 10 min substrate treatment timescorresponding approximately to the optimum deposition conditionsidentified by Fernandes, et. al.⁴⁶ The films exhibited broadenedabsorbance spectra consistent with previous reports⁴⁶ in FIG. 1B forfilms on quartz as functions of the number of PMo₁₁V/PR bilayers, n, forn=1-20. However, absorbance changes exhibited two linear regions withincreasing numbers of PMo₁₁V/PR bilayers, rather than a single nearlylinear growth behavior, as shown in the FIG. 1C. In addition, the ˜535nm absorbance of the Quartz/PEI/(PMo₁₁V/PR)₂₀ film was ˜0.38, which issomewhat smaller than the ˜0.46 value found for the corresponding filmin the literature.⁴⁶

In order to better understand the nature of these differences, thepotential effects of the PEI layer on film deposition were examined.Films of structure Quartz/PEI/(PMo₁₁V/PR)₂₀ were deposited on freshPEI-coated quartz and PEI-coated quartz that had been stored in sealedCoplin jars for up to 1 week at room temperature. Absorbance values at535 nm obtained for the films were 0.38 and 0.36, respectively,indicating that any changes in PEI conformation and degree ofprotonation or potential PEI reactions involving carbon dioxide⁵³ orother species present in the ambient atmosphere did not significantlyaffect film deposition.

Potential effects due to the preparation of the Na₄H[(VMo₁₁)O₄₀] (i.e.,PVMo₁₁) species were examined. Films were prepared usingNa₄H[(VMo₁₁)O₄₀] that was freeze dried immediately after preparation,preserving the blue color corresponding to the V^(IV) form of thematerial in the resulting solid.⁴⁷ Dissolution of this material in pH4/0.1 M acetate buffer (aq) resulted in a blue solution, which wasimmediately used to deposit the films. The solution color slowly changedto orange-brown, corresponding to the V^(V) form of the material, withincreasing time at room temperature. Completion of the color changecorresponded approximately to the time required to deposit 5-7 PMo₁₁V/PRbilayer, suggesting that the multi-slope behavior and breakpointobserved in FIG. 1C were related to the change in oxidation state of thePMo₁₁V species. Because the V^(IV) form of the PMo₁₁V possesses anadditional unit of negative charge compared to the V^(V) form,differences in packing density with PR, film hydration, and inclusion ofsolution anions such as OAc⁻ are expected to occur during filmdeposition. These factors in turn will affect film growth rate andabsorbance by altering the amounts of PMo₁₁V and PR deposited.

Chlorate Electrochemistry—FIG. 2A shows a series of cyclic voltammogramsfor a n=6 PVMo₁₁/PR bilayer film on ITO in aerated 0.10 M sodium acetatepH 2.5 (aq) solution as a function of scan rate, together with a plot ofthe scan rate dependence of the current at E_(pc)=−0.250 V andE_(pa)=−0.203 V. Cathodic and anodic peak currents vary linearly withscan rate in FIG. 2B, as expected for surface confined processes.⁵⁴ Theindividual peaks observed in the −0.45≤E≤0.80 V potential range scannedare attributed to the polyoxometalate, with noticeable differences tothe literature values⁴⁶ most likely do to different conditions e.g., ITOvs. GCE, pH ˜2.5/0.10 M sodium acetate electrolyte vs. pH ˜4/0.10 Msodium acetate (aq) buffer electrolyte, and PR-terminated vs.PVMo₁₁-terminated films, respectively.

TABLE 1 Electrochemical parameters for n = 6 PVMo₁₁/PR bilayer film onITO This work^(a) Literature^(b) Peak E_(pc) E_(pa) (ΔE_(p)) E_(pc)(ΔE_(p)) 1 0.290 0.355^(c) 0.065 0.367 0.06 2a 0.025 0.045 0.020 0.0980.029-0.09 2b −0.080 −0.055 0.025 — — 3 −0.250 −0.203 0.047 −0.1220.029-0.09 4 −0.400 −0.360 0.040 −0.266 0.029-0.09 ^(a)Scan rate = 100mV · s⁻¹ ^(b)Reference⁴⁶ ^(c)Shoulder at 0.53 V vs. Ag/AgCl

The peak positions and assignments from the cyclic voltammograms arelisted in Table 1. The cathodic peak potential (E_(pc)) of Peak 1 hasshifted from E_(pc)=0.290 to 0.323 V compared to the literature. Peak 2has split from a broad peak with E_(pc)=0.098 V to two peaks withE_(pc)=0.035 and −0.068 V. Peak 3 has shifted from E_(pc)=0.122 to−0.250 V, and Peak 4 has shifted from E_(pc)=−0.266 to E_(pc)=−0.44 V.No redox processes are observed for the PR species. Peak-to-peakseparations (ΔE_(p)) of ˜0.06 V are observed for the V-based waveassigned to Peak 1, whereas ΔE_(p) values between 0.02 to 0.05 V arenoted for Mo-based waves assigned for Peaks 2 thru 4. Integration of thecathodic and anodic waves of Peak 1 yields a polyoxometalate surfacecoverage of Γ=0.16±0.05 nmole·cm⁻² for a film comprising n=6 bilayers.

FIG. 2A shows that redox processes associated only with the PVMo₁₁component of the film are observed, even though the CV was carried outusing aerated electrolyte solutions. This is further illustrated in FIG.3, where essentially identical voltammograms are obtained for thecomposite electrode in aerated and Ar-purged buffer solutions. Likewise,the addition of TNT at trace levels (0.01 mg·mL⁻¹) to the solutionprovides no additional signals at the composite electrode in FIG. 3,consistent with the selective nature of the modified electrode.

Results of aging films and film deposition reproducibility can be seenin FIG. 2B, which shows cyclic voltammograms of ITO and PVMo₁₁/PRbilayers (n=5) on ITO comparing different films aged at different times.Current measurements for the two films aged 8 weeks are identical withinexperimental error to the current obtained for the film aged 5 weeks,indicating that any changes in film structure are complete by 8 weeksand the films have reached equilibrium by that time. Identical currentswithin experimental error observed for the two 8 week old filmsdemonstrate and confirm film deposition reproducibly.

FIG. 4A illustrates the effect of the presence of chlorate in thesolution on the CV of the 1 week old n=6 PVMo₁₁/PR bilayer film on ITO.A strong catalytic wave is observed at the E_(pc4) Mo-based wave,corresponding to the pH dependent catalyzed reduction of chlorate tochloride according to eq. (1) and (2):

PV^(IV)Mo₄ ^(V)Mo₇ ^(VI)O₄₀ ¹⁰⁻+2e ⁻→PV^(IV)Mo₆ ^(V)Mo₅ ^(VI)O₄₀ ¹²⁻(E_(pc4)=−0.40 V vs. Ag/AgCl)   (1)

3PV^(IV)Mo₆ ^(V)Mo₅ ^(VI)O₄₀ ¹²⁻+ClO₃ ⁻+6H⁺→3PV^(IV)Mo₄ ^(V)Mo₇ ^(VI)O₄₀¹⁰⁻+3H₂O+Cl⁻  (2a)

3PV^(IV)Mo₆ ^(V)Mo₅ ^(VI)O₄₀ ¹²⁻+ClO₃ ³¹+3H₂O→3PV^(IV)Mo₄ ^(V)Mo₇^(VI)O₄₀ ¹⁰⁻+6OH⁻+Cl⁻  (2b)

Two one-electron reductions of Mo^(VI) components of the PVMo₁₁ speciesin the film to the me oxidation state produce a highly reduced productin eq. (1) capable of reducing chlorate to chloride under acid or basicconditions in eqs. (2). The oxidized PV^(IV)Mo₄ ^(V)Mo₇ ^(VI)O₄₀ ¹⁰⁻species produced as the product in eqs. (2) is identically the samespecies serving as reactant in eq. (1). At the applied electrodepotential designated by E_(pc4), it is immediately reduced once again toPV^(IV)Mo₆ ^(V)Mo₅ ^(VI)O₄₀ ¹²⁻ and reacts with additional chlorate tocontinue the catalytic cycle.

Catalytic reduction current increases with [ClO₃ ⁻] in solution in FIG.4A, resulting in a linear calibration curve in FIG. 4B. The limit ofdetection (LOD) was calculated using the equation LOD=3σ/m where “σ” isthe standard deviation of signal at the lowest concentration tested and“m” is the slope of the calibration curve. From FIG. 3B, the LOD=220 μMand the sensitivity=0.022±0.002 μA/μM of KClO₃. In small volumeelectrochemical cell of 100 μL, this would correspond to 2.6 μg ofKClO₃, which is sufficient to provide a “yes/no” answer for detection ofbulk chlorate salts in the field for surveillance of bomb makingactivities.

Electrode Performance Optimization—Having completed the initialcharacterization of the electrodes and demonstrated their ability tocatalyze electroreduction of chlorate, the next step was to determineconditions for optimum performance using a Taguchi L16 array. Thefactors (i.e., variables) examined included number of PMo₁₁V/PR bilayerspresent on the electrode (L), solution acidity/pH (H), solution [ClO₃ ⁻](C; μM), CV scan rate (S; mV·s⁻¹), and PMo₁₁V/PR film age (A; weeks).Each factor was examined at k=4 levels at the following factor/levelvalues as follows: L/1=3, L/2=4, L/3=5, and L/4=6 PMo₁₁V/PR bilayers;H/1=1.32±0.02, H/2=1.80±0.01, H/3=2.31±0.01, and H/4=2.85±0.03 pH units;C/1=250, C/2=500, C/3=750, and C4=1000 μM chlorate; S/1=50, S/2=100,S/3=150, and S/4=200 mV·s⁻¹ scan rate; A/1=1, A/2=2, A/3=5, and A/4=8weeks aging.

TABLE 2 Taguchi L16 Array Expt. Variables Net Current, i (μA) AverageStandard S/N No. L H C S A i₁ i₂ i₃ i_(ave) (μA) Deviation (σ) Ratio 1 11 1 1 1 8.69 6.89 6.06 7.21 0.87 16.88 2 1 2 2 2 2 17.49 14.77 13.5715.28 1.31 23.54 3 1 3 3 3 3 16.62 15.47 14.98 15.69 0.55 23.89 4 1 4 44 4 49.26 40.56 37.20 42.34 4.12 32.36 5 2 1 2 3 4 41.65 34.86 33.5836.70 3.05 31.18 6 2 2 1 4 3 43.12 32.82 29.96 35.30 4.74 30.65 7 2 3 41 2 18.94 15.75 14.18 16.29 1.56 24.05 8 2 4 3 2 1 42.59 36.58 33.7037.62 2.93 31.39 9 3 1 3 4 2 84.37 72.68 70.22 75.76 5.28 37.51 10 3 2 43 1 121.39 110.27 104.17 111.95 5.54 40.93 11 3 3 1 2 4 30.97 25.4223.06 26.48 2.66 28.27 12 3 4 2 1 3 22.44 19.34 17.62 19.80 1.55 25.8113 4 1 4 2 3 39.05 28.03 24.22 30.43 5.16 29.17 14 4 2 3 1 4 14.93 12.4611.43 12.94 1.18 22.08 15 4 3 2 4 1 100.70 81.71 73.93 85.45 9.07 38.4216 4 4 1 3 2 82.25 68.55 62.32 71.04 6.63 36.86

Table 2 summarizes the Taguchi L16 array, with each row indicating anexperiment at conditions corresponding to the coded values for eachfactor shown. Net currents obtained for each of x=3 replicates aftersubtraction of background currents and corrections for capacitiveeffects are shown, together with the average current and its standarddeviation for each of m=16 total experiments. The S/N ratio is definedfor optimization related to current maximization by summing the inversesquares of each net current measurement, dividing by the number ofreplicate measurements, “x=3”, taking the base 10 logarithm of the valueobtained, and finally multiplying that value by −10.

The mean S/N ratio of each factor, F, defined at each level, “i”, ofthat factor is given by M_(i) ^(F). The M_(i) ^(F) are calculated bysumming the four S/N values corresponding to a given fixed level forthat factor and dividing by k=4 (i.e., the number of levels availablefor each factor). For example, factor A at level 2 (i.e., A/2) occurs inexperiments 2, 7, 9, and 16, corresponding to S/N ratios of 23.54,24.04, 37.51, and 36.86 in Table 2. These values are shown in the arrayin Table 3 as entries in columns j=1-4 in the A/2 row. Their sum is121.96, which when divided by k=4 levels for each variable yields theM_(i) ^(F)=M₂ ^(A)=30.49 value shown in Table 3 below. Population of theremaining M_(i) ^(F) in analogous fashion completes Table 3.

Conditions associated with the maximum current response for theelectrode correspond to the levels of each factor having the largestM_(i) ^(F) value. In this case, the maximum current response correspondsto the L/3, H/4, C/4, S/4, A/1 factor level combination indicated inbold italic text in Table 3. That is, an electrode comprising L=5bilayers aged A=1 week in a solution containing C=1000 μM chlorate atH=2.85 pH scanned at S=200 mV·s⁻¹ provides the largest current response.

TABLE 3 Mean S/N Ratios for Each Factor Factor/ [(S/N)_(i) ^(F)]_(j)Experimental Level j = 1 j = 2 j = 3 j = 4 M_(i) ^(F) Conditions L/116.88 23.54 23.89 32.36 24.17 L/1 = 3 bilayers L/2 31.18 30.65 24.0531.39 29.32 L/2 = 4 bilayers

 = 

L/4 29.17 22.08 38.42 36.86 31.63 L/4 = 6 bilayers H/1 16.88 31.18 37.5129.17 28.68 H/1 = 1.32 ± 0.02 pH H/2 23.54 30.65 40.93 22.08 29.30 H/2 =1.80 ± 0.01 pH H/3 23.89 24.05 28.27 38.42 28.66 H/3 = 2.31 ± 0.01 pH

 = 

C/1 16.88 30.65 28.27 36.86 28.17 C/1 = 250 μM ClO₃ ⁻ C/2 23.54 31.1825.81 38.42 29.74 C/2 = 500 μM ClO₃ ⁻ C/3 23.89 31.39 37.51 22.08 28.72C/3 = 750 μM ClO₃ ⁻

 = 

S/1 16.88 24.05 25.81 22.08 22.20 S/1 = 50 mV · s⁻¹ S/2 23.54 31.3928.27 29.17 28.09 S/2 = 100 mV · s⁻¹ S/3 23.89 31.18 40.93 36.86 33.21S/3 = 150 mV · s⁻¹

 =

 =

A/2 23.54 24.05 37.51 36.86 30.49 A/2 = 2 weeks aged A/3 23.89 30.6525.81 29.17 27.38 A/3 = 5 weeks aged A/4 32.36 31.18 28.27 22.08 28.47A/4 = 8 weeks aged

From the Taguchi analysis, the relative contributions and influences ofthe L, H, C, S, and A factors on system performance were estimated via astandard ANOVA calculation. The percentage contribution of each factor,F, at level “i”, R_(i) ^(F), is first calculated. The calculation isidentical to that performed for the M_(i) ^(F) of Table 3, with theexception that Ave. Current Results, rather than individual S/N, fromTable 2 are used in the calculation. The results are shown in Table 4below.

TABLE R_(i) ^(F) Calculations [(R_(A))_(i) ^(F)]_(j) Factor/Level j = 1j = 2 j = 3 j = 4 R_(i) ^(F) L/1 7.21443 15.2766 15.6903 42.3397 20.1303L/2 36.7 35.3004 16.2874 37.6199 31.4769 L/3 75.7572 111.947 26.481419.8011 58.4967 L/4 30.4321 12.9367 85.4472 71.0404 49.9641 H/1 7.2144336.7 75.7572 30.4321 37.5259 H/2 15.2766 35.3004 111.947 12.9367 43.8652H/3 15.6903 16.2874 26.4814 85.4472 35.9766 H/4 42.3397 37.6199 19.801171.0404 42.7003 C/1 7.21443 35.3004 26.4814 71.0404 35.0092 C/2 15.276636.7 19.8011 85.4472 39.3062 C/3 15.6903 37.6199 75.7572 12.9367 35.501C/4 42.3397 16.2874 111.947 30.4321 50.2515 S/1 7.21443 16.2874 19.801112.9367 14.0599 S/2 15.2766 37.6199 26.4814 30.4321 27.4525 S/3 15.690336.7 111.947 71.0404 58.8444 S/4 42.3397 35.3004 75.7572 85.4472 59.7111A/1 7.21443 37.6199 111.947 85.4472 60.5571 A/2 15.2766 16.2874 75.757271.0404 44.5904 A/3 15.6903 35.3004 19.8011 30.4321 25.306 A/4 42.339736.7 26.4814 12.9367 29.6144

TABLE 5 Summary of SS_(F) Calculations Factor SS_(F) Values DF_(F)Values L SS_(L) = 10906.3 DF_(L) = 3 H SS_(H) = 534.461 DF_(H) = 3 CSS_(C) = 1808.68 DF_(C) = 3 S SS_(S) = 18887.6 DF_(S) = 3 A SS_(A) =9209.26 DF_(A) = 3

Completion of the ANOVA calculations required knowledge of the total sumof squares, SS_(T). Table 6 below shows the squared results for each netcurrent measurement shown in Table 2 used to calculate SS_(T) via thefollowing equation:

SS _(T)=Σ_(j=1) ^(m)(Σ_(i=1) ^(x) R _(i) ²)_(j)−(mx)R _(T) ²=42647.19984  (6)

TABLE 6 Squares of the Net Currents Expt. Factors Squares of NetCurrents No. L H C S A i₁ ² i₂ ² i₃ ² 1 1 1 1 1 1 75.5874 47.480436.7066 2 1 2 2 2 2 305.97 218.212 184.031 3 1 3 3 3 3 276.234 239.265224.475 4 1 4 4 4 4 2426.45 1645.09 1383.86 5 2 1 2 3 4 1735.06 1215.371127.86 6 2 2 1 4 3 1859.16 1077.06 897.883 7 2 3 4 1 2 358.538 247.955201.087 8 2 4 3 2 1 1813.6 1337.92 1135.39 9 3 1 3 4 2 7118.47 5281.744931.54 10 3 2 4 3 1 14736.3 12160.5 10852 11 3 3 1 2 4 958.85 646.202531.694 12 3 4 2 1 3 503.558 374.07 310.549 13 4 1 4 2 3 1524.77 785.839586.38 14 4 2 3 1 4 222.782 155.174 130.582 15 4 3 2 4 1 10140.7 6676.835465.47 16 4 4 1 3 2 6765.59 4699.01 3883.61

Using the SS_(T) and SS_(F) values, the sum of the squares of theexperimental error, SS_(ERROR), and the variance of the error,V_(ERROR), were calculated from the following equations:

SS _(ERROR) =SS _(T)−Σ_(F=A) ^(F)(SS _(F))=1300.907799   (7)

V _(ERROR)=(SS _(T)−Σ_(F=A) ^(E) SS _(F))/(m(x−1))=40.65336873   (8)

The relative contribution of factor F to the response is calculatedusing the following equation:

ρ_(F)=100(SS _(F)−(DF _(F))(V _(ER)))/SS _(T)   (9)

Values of ρ_(F) for each factor are summarized Table 7:

TABLE 7 ρ_(F) Values Factor ρ_(F) L ρ_(A) = 25.2873 H ρ_(B) = 0.96724 Cρ_(C) = 3.95505 S ρ_(D) = 44.002 A ρ_(E) = 21.3081

The M_(i) ^(F) from Table 3 and the SS_(F) values from Table 5 for thefactors are summarized in the Table 8 below, together with the ranges ofthe M_(i) ^(F) for each factor. The relative contributions of eachfactor correspond to decreasing order of the SS_(F)'s and ranges.

TABLE 8 Significance Contributions of the Factors Factors Levels L H C SA 1 24.1663 28.684 28.1656 22.2047 31.9048 2 29.3194 29.2999 29.736928.0899 30.4899 3 33.1273 28.6581 28.7153 33.2146 27.3793 4 31.632331.6034 31.6275 34.7361 28.4713 Range^(a) 8.96104 2.94531 3.4618312.5314 4.52549 SS_(F) 10906.3 534.461 1808.68 18887.6 9209.26 Rank 2 54 1 3 (Range)^(b) ^(a)Range = M_(i) ^(F) (maximum)-M_(i) ^(F) (minimum)^(b)Significance Contribution: S > L > A > C > H

Therefore, ANOVA calculations using the measured net current values inTable 2 identify scan rate, S, as the factor most contributing to thecurrent response, followed in decreasing order by the variables L, A, C,and H.

Further examination of the M_(i) ^(F) values and their variations amongthe levels studied for each factor provide additional insight into thenature of the electrode and the chlorate electroreduction process. Forexample, Table 3 identifies a film having L/3=5 bilayers as the mostefficient in promoting maximum current response. The thinner 3 and 4bilayer films and the thicker 6 bilayer film provide lower responses, asindicated by their M_(i) ^(L) values. The proportional currentreductions noted for the 3 and 4 bilayer films can be ascribed to thepresence of less redox active PMo11V in these thinner films. Incontrast, the 6 bilayer film contains the most PMo11V yet also exhibitsdecreased current compared to the 5 bilayer film. The 6 bilayer filmcorresponds approximately to the absorbance break point in FIG. 1B, atwhich differences in film packing ostensibly occur as oxidation of theblue V^(IV) form of the PMo₁₁V solution species deposited to the orangeV^(V) form is completed. Associated changes in packing density andinternal film structure, together with increased separation between theunderlying electrode and solution as film thickness increases, resultingin lowered film permeability and currents provide an explanation for theobservations consistent with similar behavior noted for correspondingthick (PR/PMo₁₁V)_(n) films (n≥5) elsewhere.⁴⁶

Not surprisingly, film aging also significantly affects the currentresponse. The largest M_(i) ^(A) in Table 3 is observed using the filmsaged 1 week. Thereafter, current response falls until the film has aged5 weeks, then recovers slightly (˜10%) for the 8 week old film. Thisbehavior indicates some restructuring of the films as they age, leadingto changes in permeability and mechanical properties reflected in thecurrent response. Similar behaviors have been noted forparaquat-silicate^(49,55,56) and polyelectrolyte multilayer⁵⁷⁻⁶¹ films,in which components are usually deposited in kinetically trappedconformations that slowly relax via chemical or physical processes,respectively, to their thermodynamically stable equilibriumconformations as the films age. That similar phenomena related tochanges in internal structure occur in the films is also supported byobservations of slight changes in film absorbance noted by Fernandes,et. al.⁴⁶ in analogous (PR/PMo₁₁V)_(n) films following overnightdrying/re-wetting cycles during film deposition.

The pH dependence in Table 3 indicates that current is generallyenhanced by increasing the solution pH, although the changes in M_(i)^(H) are non-monotonic suggesting that factor H may be stronglyinfluenced by one or more other factors in the system. Nevertheless, theH/4=2.85 pH solution provides the largest response. This contrasts withbehaviors noted for chlorate electroreduction by otherpolyoxometalates,^(21,62,63) for which reduction is generally morefavorable at lower pH. Chlorate electroreduction can occur in bothacidic and basic solutions,^(6,40) with reduction in acidic solutionthermodynamically much more favorable:

ClO₃ ⁻+6H⁺+6e ⁻↔Cl⁻+3H₂O (E⁰=1.45 V vs. S.H.E.; acidic pH)   (10)

ClO₃ ⁻+3H₂O+6e ⁻↔Cl⁻+6OH⁻ (E⁰=0.62 V vs. S.H.E.; basic pH)   (11)

The fact that chlorate electroreduction for the films is somewhat morefavorable at higher pH suggests that reduction may occur primarily viaeq. (11), rather than eq. (10). Contributions to chlorate reduction viaeq. (11) are supported by the structure of the films, which areterminated by a layer of cationic, hydrophobic PR species also containedwithin the film interior. The PR component is expected to provide anelectrostatic and hydrophobic barrier inhibiting entry of protons, whichpossess a high charge density and highly structured water solvent shell,into the film. In contrast, for a neutral molecule such as water orpoorly solvated chaotropic anions⁶⁴ such as ClO₃ ⁻ or Cl⁻, which possesszero or favorable (i.e., anionic) lower charge densities, respectively,entry required to complete the electroreduction is more facile.⁶⁵

For the S and C factors changes in current response are generallyconsistent with expectations for CV measurements. For example, CVcurrents increase with scan rate, S, which reflects the change indriving force for the reaction at the electrode. M_(i) ^(S) values inTable 3 increase in a monotonic fashion with S as expected, with maximumcurrent observed for the fastest scan rate, S/4. In similar manner, thelargest current response occurs at the highest chlorate concentration(C/4=1000 μM), consistent with the current vs. [ClO₃ ⁻] response alreadynoted in FIG. 4B. The non-monotonic change in M_(i) ^(C) on proceedingfrom level i=1 to i=4, however, is analogous to that observed for pHwith M_(i) ^(H), again suggesting that one or more other factors, suchas film age, may interact with and alter the chlorate concentrationeffect. These non-monotonic variations in M_(i) ^(C) and M_(i) ^(H) arediscussed further below.

Electrode Performance Model—In conjunction with the Taguchi study,experiments were also conducted using a two-level full factorial designto obtain a mathematical model for the system. Given the age effectresults noted for the films from the Taguchi analysis, study wasdirected to films aged 8 weeks that had reached their thermodynamicequilibrium. Therefore the design probed only the effects of thefactors, L, H, C, and S, each examined at two levels using coded levels(i.e., ±1) for each factor as defined by eq. (12):

p=2(q−a)/(b−a)−1   (12)

In eq. (12), a and b represent the low and high values for a givenfactor associated with its −1 and +1 coded values in the design; q isany intermediate value for the factor within the range a≤q≤b; and p isits corresponding value linearly mapped onto the −1≤p≤+1 range. Thevalues of L=−1≡3 bilayers and L=+1≡5 bilayers were chosen to furtherinvestigate the region of monotonic increase in current with increasedbilayer number identified by the Taguchi design. For the solution pHfactor, H was defined directly in terms of [H⁺] in solution, rather thanthe pH values associated with each solution, with H=−1≡0.0014 M HCl(i.e., pH 2.85) and H=+1≡0.0479 M HCl (i.e., pH 1.32). For the remainingfactors, coded levels corresponded to the minimum and maximum values ofeach factor used in the Taguchi design as follows: C=−1≡250 μM ClO₃ ⁻and C=+1≡1000 μM ClO₃ ⁻; and S=−1≡50 mV·s⁻¹ and S=+1≡200 mV·s⁻¹.

A standard order design matrix of the 2⁴=16 possible combinations offactor levels was prepared as shown in Table 9, which also summarizesthe net current measurements and provides the average current observedand its standard deviation for each experiment comprising the two-levelfull factorial design for the 8 week old film electrodes

TABLE 9 Standard Order Design Matrix and Measured Net Currents Expt.Factors Net Currents (μA) No.^(a) L H C S i₁ i₂ i₃ i_(ave) (μA)^(c)σ_(i)(μA)^(d) 1  −1^(b) −1 −1 −1 13.18 10.68 9.57 11.14 1.85 2 +1 −1 −1−1 20.97 16.86 15.14 17.66 3.00 3 −1 +1 −1 −1 8.23 6.69 6.04 6.98 1.12 4+1 +1 −1 −1 12.20 9.85 8.87 10.31 1.71 5 −1 −1 +1 −1 15.99 13.26 11.9913.74 2.04 6 +1 −1 +1 −1 25.49 20.93 18.99 21.81 3.34 7 −1 +1 +1 −111.66 9.92 9.06 10.21 1.32 8 +1 +1 +1 −1 25.41 19.85 17.83 21.03 3.93 9−1 −1 −1 +1 50.23 39.93 36.23 42.13 7.25 10 +1 −1 −1 +1 71.22 58.4553.28 60.98 9.24 11 −1 +1 −1 +1 26.25 20.82 19.17 22.08 3.70 12 +1 +1 −1+1 42.20 33.71 31.04 35.65 5.83 13 −1 −1 +1 +1 52.95 43.11 39.27 45.117.06 14 +1 −1 +1 +1 80.70 67.15 61.37 69.74 9.92 15 −1 +1 +1 +1 34.7129.42 27.59 30.57 3.70 16 +1 +1 +1 +1 58.58 47.53 43.90 50.01 7.65^(a)Experiment number. Each row in the matrix specifies coded values pereq. (12) for each factor used in that experiment as specified in theFactors column. ^(b)Coded ± 1 level for the factors. ^(c)Average currentcalculated from net currents as i_(ave) = (i₁ + i₂ + i₃)/3 ^(d)Standarddeviation.

Table 10 summarizes the calculation of the Effects from the averagecurrents measured for each experiment using Yates' Algorithm.⁵² Effectsare identified with the appropriate factor or combination of factors.Identical values obtained for a “Squares Check” for each column of theYates matrix confirms that the calculations are correct.

TABLE 10 Summary of Yates' Algorithm Calculation of Effects Expt.i_(ave) Column Number, y^(a) Effect Effect No. (μA) 1 2 3 4 DivisorE^(b) ID^(c) 1 11.14393 28.80073 46.092 112.8869 469.1537 16 29.3221Average 2 17.6568 17.29127 66.7949 356.2668 105.1995 8 13.14993 L 36.984133 35.5498 160.8378 28.7131 −95.4592 8 −11.9324 H 4 10.3071331.2451 195.4289 76.48637 −10.9119 8 −1.363992 LH 5 13.7442 103.10719.835867 −15.8142 55.294 8 6.91175 C 6 21.8056 57.7307 18.87723 −79.64520.67393 8 2.584242 LC 7 10.21463 114.8488 32.4269 −0.43543 18.3126 82.28908 HC 8 21.03047 80.58017 44.05947 −10.4765 6.032667 8 0.75408 LHC9 42.12623 6.512867 −11.5095 20.7029 243.3799 8 30.42248 S 10 60.98093.323 −4.3047 34.5911 47.77327 8 5.971658 LS 11 22.07923 8.0614 −45.37649.041367 −63.8309 8 −7.978858 HS 12 35.65147 10.81583 −34.2686 11.63257−10.0411 8 −1.255133 LHS 13 45.111 18.85467 −3.18987 7.204767 13.8882 81.736025 CS 14 69.73777 13.57223 2.754433 11.10783 2.5912 8 0.3239 LCS15 30.57373 24.62677 −5.28243 5.9443 3.903067 8 0.48788 HCS 16 50.0064319.4327 −5.19407 0.088367 −5.85593 8 −0.731992 LHCS 19387.54 38775.0777550.14 155100.3 310200.6 =Sum of Squares = Q^(d) 19387.54 19387.5419387.54 19387.54 19387.54 =Q/2^(y) (Squares Check) ^(a)Yates' Algorithmmatrix elements. ^(b)Effect calculated by dividing each element ofColumn 4 by the corresponding Divisor. ^(c)Factor or factor interactionassociated with each Effect. ^(d)Sum of the squares of each entry incolumn y.

The probability of each Effect occurring solely due to random error wascomputed using eq. (13):

P _(j)=100(j−½)/15 (j=1-15 Effects)   (13)

In eq. (13), P_(j) is the probability that the j^(th) Effect isstatistically non-significant and occurs simply due to random error. TheEffects are ordered from most negative to most positive and assigned theappropriate P_(j) value from eq. (13), as summarized in Table 11 below.

The normal probability plot of the Effects vs. P_(j) data from Table 11is shown in FIG. 5. Nine points comprise the straight line section ofthe plot, signifying Effects that are statistically non-significant andcan be attributed to random error. The remaining 6 points deviating fromthe straight line portion of the plot represent Effects that providestatistically significant contributions to the observed currentresponse. These comprise the Effects due to the L, S, H, and C factorsand the HS and LS factor interactions.

TABLE 11 Two-level Full Factorial Design Effects & Probabilities Due toRandom Error Average Proba- Expt. Factors (F)^(a) Current, Effect Effectbility Order No. L H C S i_(ave) (μA)^(b) ID^(c) (E)^(d) (P_(j))^(e)(j)^(f) 1 −1 −1 −1 −1 11.14 Average 29.32 — — 2 +1 −1 −1 −1 17.66 L13.15 90.000 14 3 −1 +1 −1 −1 6.98 H −11.93 3.333 1 4 +1 +1 −1 −1 10.31LH −1.36 16.667 3 5 −1 −1 +1 −1 13.74 C 6.91 83.333 13 6 +1 −1 +1 −121.80 LC 2.58 70.000 11 7 −1 +1 +1 −1 10.21 HC 2.29 63.333 10 8 +1 +1 +1−1 21.03 LHC 0.75 50.000 8 9 −1 −1 −1 +1 42.13 S 30.42 96.667 15 10 +1−1 −1 +1 60.98 LS 5.97 76.667 12 11 −1 +1 −1 +1 22.08 HS −7.98 10.000 212 +1 +1 −1 +1 35.65 LHS −1.26 23.333 4 13 −1 −1 +1 +1 45.11 CS 1.7456.667 9 14 +1 −1 +1 +1 69.74 LCS 0.32 36.667 6 15 −1 +1 +1 +1 30.57 HCS0.49 43.333 7 16 +1 +1 +1 +1 50.01 LHCS −0.73 30.000 5 ^(a)Coded factorlevels defined in eq. (12). ^(b)Average of 3 net current measurementsfrom Table 9. ^(c)Identification of factors and/or factor interactionsassociated with calculated Effects. ^(d)Effects calculated using theYates' Algorithm in Table 10. ^(e)Probability of the Effect being duesolely to random error calculated using eq. (13). ^(f)Numerical order ofthe Effects from most negative to most positive for probability ofrandom occurrence assignment, P_(j), using eq. (13).

ANOVA calculations can be used to verify the significance of the S, L,H, C factors and HS and LS factor interactions identified in FIG. 5.Table 12 summarizes the calculations of the average net current(i_(ave)), degrees of freedom (DF) and variance (V) for each factorialexperiment, “i”. The terms are defined as follows for the “i^(th)”experiment:

(i _(ave))_(i)=(i ₁ +i ₂ +i ₃)_(i) /x (where x=3 replicate currentmeasurements per experiment)   (14)

DF _(i) =x−1=3−1=2   (15)

V _(i)=((i ₁ −i _(ave))²+(i ₂ −i _(ave))²+(i ₃ −i _(ave))²)_(i) /DF _(i)  (16)

TABLE 12 Calculation of the Current Averages, Degrees of Freedom, andVariance Expt. Factors Net Current No. (i) L H C S i₁(μA) i₂(μA) i₃(μA)i_(ave)(μA) DF V 1 −1 −1 −1 −1 13.18 10.68 9.57 11.14333 2 3.41903 2 +1−1 −1 −1 20.97 16.86 15.14 17.65667 2 8.97323 3 −1 +1 −1 −1 8.23 6.696.04 6.986667 2 1.26503 4 +1 +1 −1 −1 12.20 9.85 8.87 10.30667 2 2.928635 −1 −1 +1 −1 15.99 13.26 11.99 13.74667 2 4.17763 6 +1 −1 +1 −1 25.4920.93 18.99 21.80333 2 11.1345 7 −1 +1 +1 −1 11.66 9.92 9.06 10.21333 21.75453 8 +1 +1 +1 −1 25.41 19.85 17.83 21.03 2 15.4084 9 −1 −1 −1 +150.23 39.93 36.23 42.13 2 52.63 10 +1 −1 −1 +1 71.22 58.45 53.2860.98333 2 85.2742 11 −1 +1 −1 +1 26.25 20.82 19.17 22.08 2 13.7223 12+1 +1 −1 +1 42.20 33.71 31.04 35.65 2 33.9591 13 −1 −1 +1 +1 52.95 43.1139.27 45.11 2 49.7856 14 +1 −1 +1 +1 80.70 67.15 61.37 69.74 2 98.443315 −1 +1 +1 +1 34.71 29.42 27.59 30.57333 2 13.6712 16 +1 +1 +1 +1 58.5847.53 43.90 50.00333 2 58.4636

From Table 12, the total number of current measurements for 16experiments each replicated 3 times is N=48. The grand average responseand the sum of the degrees of freedom are given by the following:

Grand Average=29.32=Σ_(i=1) ¹⁶(i ₁ +i ₂ +i ₃)_(i) /N   (17)

DF _(T)=32=Σ_(i=1) ¹⁶(DF)_(i)   (18)

Therefore, the mean square error (MSE) for the system is given by:

MSE=28.4382=Σ_(i=1) ¹⁶(DF)_(i)·(V)_(i) /DF _(T)   (19)

Table 13 summarizes the (MSB)_(i) values calculated from each Effect,E_(i).

TABLE 13 MSB Calculations Effect Effect ID E_(i) E_(i) ² N (MSB)_(i) L13.1499 172.921 48 2075.05 H −11.932 142.382 48 1708.59 LH −1.3641.86047 48 22.3257 C 6.91175 47.7723 48 573.267 LC 2.58424 6.6783 4880.1397 HC 2.28908 5.23986 48 62.8784 LHC 0.75408 0.56864 48 6.8237 S30.4225 925.527 48 11106.3 LS 5.97166 35.6607 48 427.928 HS −7.978963.6622 48 763.946 LHS −1.2551 1.57536 48 18.9043 CS 1.73603 3.01378 4836.1654 LCS 0.3239 0.10491 48 1.25893 HCS 0.48788 0.23803 48 2.85636LHCS −0.732 0.53581 48 6.42974

There are DF_(MSE)=32 and DF_(MSB)=1 for this factorial design. In orderto test the hypothesis (H₁) that a given Effect is statisticallysignificant vs. the null hypothesis (H₀) that the Effect is notstatistically significant, the (MSB)_(i) is compared to the MSE bydefining the (F₀)_(i) ratio:

(F ₀)_(i)=(MSB)_(i) /MSE   (21)

The (F₀)_(i) are compared to the corresponding F_(c) value, taken froman F-distribution table, that would be expected if a given E_(i) is dueto random error. For the systems, the appropriate F_(C) is of the formF_(C)(0.999, DF_(MSB), DF_(MSE))=F_(C)(0.999, 1, 32), where 0.999represents the confidence limit for the significance evaluation. TheF-distribution table does not include values for DF_(MSE)=32, so themore conservative available F_(C)(0.999, 1, 30)=13.3 is used instead.The ratio (F₀)_(i)/F_(C) is formed for each effect, E_(i). Values of(F₀)_(i)/F_(C)>1 correspond to statistically significant E_(i), at the0.999 confidence level, whereas values less than one correspond to E_(i)associated with random error. Table 14 summarizes these calculation.

TABLE 14 ANOVA Identification of Statistically Significant EffectsF_(C)(0.999, Significance Effect (MSB)_(i) MSE (F₀)_(i) 1, 30)(F₀)_(i)/F_(C) (F₀)_(i)/F_(C) > 1 ID 2075.05 28.4382 72.9671 13.35.48625

1708.59 28.4382 60.0808 13.3 4.51735

22.3257 28.4382 0.78506 13.3 0.05903 No LH 573.267 28.4382 20.1584 13.31.51567

80.1397 28.4382 2.81803 13.3 0.21188 No LC 62.8784 28.4382 2.21106 13.30.16624 No HC 6.8237 28.4382 0.23995 13.3 0.01804 No LHC 11106.3 28.4382390.543 13.3 29.3642

427.928 28.4382 15.0477 13.3 1.13141

763.946 28.4382 26.8634 13.3 2.01981

18.9043 28.4382 0.66475 13.3 0.04998 No LHS 36.1654 28.4382 1.27172 13.30.09562 No CS 1.25893 28.4382 0.04427 13.3 0.00333 No LCS 2.8563628.4382 0.10044 13.3 0.00755 No HCS 6.42974 28.4382 0.2261 13.3 0.017 NoLHCS

Statistically significant effects at the 0.999 confidence level areshown in bold italic typeface in Table 14. The L, H, C, S, HS, and LSEffects identified as statistically significant in Table 14 areidentical to and confirm those identified graphically in FIG. 5.

The results of the two-level full factorial study are generallyconsistent with those of the Taguchi design. The largest Effects arethose due to the S (i.e., 30.42) and L (i.e., 13.15) factors, whichprovide positive contributions to the observed current as their valuesincrease within the range of values studied. The contribution of the Hfactor (i.e., −11.93) is negative,⁶⁶ with increasing acidity (i.e.,lower pH) diminishing the current. These Effects are augmented bysmaller (relative to the S and L Effects) antagonistic HS (i.e., −7.98)and synergistic LS (i.e., 5.97) interactions that further alter currentresponse in the system.

The relative contribution of the H (i.e., −11.93) factor exceeds that ofthe C (i.e., 6.91) factor in Table 11, in opposition to the ANOVAresults from the Taguchi design. However, the Taguchi design includesthe film age factor, A, whereas film age was not examined in thetwo-level full factorial design. This behavior suggests that currentcontributions from factors C and/or H are affected by film age. Factor Cappears independently of interactions with other factors in FIG. 5,suggesting that a CA interaction may be responsible at least in part forthe non-monotonic behavior of the M_(i) ^(C) in the Taguchi design ofTable 3. Such behavior is consistent with previous observations ofcontributions of anion-film age interactions on current responses inparaquat-silicate thin film electrodes.^(55,56) Interpretation of thenon-monotonic behavior of the M_(i) ^(H) in Table 3, however, iscomplicated by the combined effects of HA interactions, if any, and theHS interaction noted in FIG. 5, which is of significant magnituderelative to the H Effect (i.e., −7.98 vs. −11.93, respectively).

The results from FIG. 5 support a model of the general form shown in eq.(22) describing the current response of the 8 week old films in terms ofthe average system response and half the sum of the products of thesignificant Effects, E, and corresponding coded factors, F:

i=Average+½Σ_(j)(E _(j))(F _(j))   (22)

Using the appropriate values for the statistically significant Effectsfrom Table 14 yields eq. (23) as a model for the 8 week old filmelectrodes in terms of the coded values of the statistically significantfactors and factor interactions identified in FIG. 5:

i=29.32+½[30.42S+13.15L−11.93H−7.98HS+6.91C+5.97LS]  (23)

For an electrode comprising a fixed number of bilayers (L) immersed in asolution of fixed acidity/pH (H) operating at a constant scan rate (S),eq. (23) correctly predicts a linear variation of current with chlorateconcentration (C) consistent with the observations in FIG. 4B.

The suitability of eq. (23) as a model equation for the electrodes canbe further tested by calculating the differences (i.e., residuals,ΔR_(i)) between the currents measured in Table 11 and those calculatedfrom eq. (23) using the coded values of each factor in each experiment.Table 15 summarizes the calculations of the residuals using the ReverseYates' Algorithm.⁵² Column 4 from Table 10 above is inverted andinserted into Table 15 as Modified Column 4 after setting entries tozero that correspond to the non-significant effects determined from FIG.5 in the main text. The Reverse Yates' matrix calculations using thevalues shown in Modified Column 4 are summarized in the “Reverse Column”portion of Table 15. The calculated value of the current, i_(calc), isobtained by dividing the value shown in Column 4 by a divisor of 16 foreach of the experiments of the two-level factorial design. Thedifference between the i_(calc) and the corresponding measured current,i_(ave), for each experiment constitutes the residual value, ΔR_(i).Identical values obtained for a “Squares Check” for each column of thereverse Yates matrix confirms that the calculations are correct.

TABLE 15 Summary of Residuals Calculations Using the Reverse Yates'Algorithm Effect Modified Reverse Column, y i_(calc) i_(ave) ΔR_(i) IDCol. 4 1 2 3 4 (μA) (μA) (μA) LHCS 0 0 0 227.3223 761.5102 47.59 50.012.42 HCS 0 0 227.3223 534.1879 455.5647 28.47 30.57 2.10 LCS 0 −63.830955.294 131.7757 1080.09 67.50 69.74 2.24 CS 0 291.1531 478.8939 323.789774.1449 48.38 45.11 −3.27 LHS 0 0 0 354.984 650.9222 40.68 35.65 −5.03HS −63.8309 55.294 131.7757 725.1063 344.9767 21.56 22.08 0.52 LS47.77327 −95.4592 55.294 259.4375 969.5023 60.59 60.98 0.39 S 243.3799574.3531 268.495 514.7074 663.5569 41.47 42.13 0.66 LHC 0 0 0 227.3223306.8657 19.18 21.03 1.85 HC 0 0 354.984 423.5999 192.0133 12.00 10.21−1.79 LC 0 −63.8309 55.294 131.7757 370.1223 23.13 21.80 −1.33 C 55.294195.6066 669.8123 213.201 255.2699 15.95 13.74 −2.21 LH 0 0 0 354.984196.2777 12.27 10.31 −1.96 H −95.4592 55.294 259.4375 614.5183 81.425275.09 6.98 1.89 L 105.1995 −95.4592 55.294 259.4375 259.5343 16.22 17.661.44 Ave. 469.1537 363.9542 459.4134 404.1194 144.6819 9.04 11.14 2.10308932.4 617864.8 1235730 2471459 4942918 =Sum of Squares = Q 308932.4308932.4 308932.4 308932.4 308932.4 =Q/2^(y) (Squares Check)

The 16 residuals, ΔR_(i), summarized in Table 15 are ordered from mostnegative to most positive and assigned probabilities of occurrence dueto random error, P_(j), using eq. (24):

P _(j)=100(j−½)/16 (j=1-16 Residuals)   (24)

Table 16 summarizes the ΔR_(i), and associated P_(j) for the j=16residuals.

TABLE 16 Model Residuals Residual Probability Order (ΔR_(i)) (P_(j)) 1−5.03 3.125 2 −3.27 9.375 3 −2.21 15.625 4 −1.96 21.875 5 −1.79 28.125 6−1.33 34.375 7 0.39 40.625 8 0.52 46.875 9 0.65 53.125 10 1.44 59.375 111.85 65.625 12 1.90 71.875 13 2.10 78.125 14 2.10 84.375 15 2.23 90.62516 2.41 96.875

The corresponding normal probability plot of the ΔR_(i) vs. P_(j) fromTable 16 is shown in FIG. 6 below. The plot is reasonably linear, with arange of ΔR_(i) (i.e., 5.03 μA≤ΔR_(i)≤2.41 μA) approximately 10% that ofthe measured net currents (i.e., 6.98 μA≤i_(ave)≤69.74 μA) from Table11. Consequently, eq. (23) provides a reasonable first approximation forthe dependence of the current response on the statistically significantEffects and factors identified by the two-level factorial design.

Applications

The electrode comprises alternating layers vanadium-substitutedphosphomolybdate polyoxometalate species and the dye, para-rosaniline.Either the para-rosaniline or the vanadium-substituted phosphomolybdatecan be the outermost layer. Optionally, the electrode is formed on asubstrate such as ITO, gold, platinum, glassy carbon, graphene, screenprinted carbon, and/or another suitable substrate. The electrode ispreferably porous. Embodiments have at least a total of four layers (twobilayers of vanadium-substituted phosphomolybdate polyoxometalate andpara-rosaniline), optionally as many as 40 total layers (20 bilayers).

In use, the electrode can be contacted directly with soil andvoltammetry performed. Preferably, the measurement is performed underdamp, conductive conditions. In embodiments, the electrode and/or soilare first wetted with a solvent, for example by spraying one or both,and/or by dipping the electrode in solvent. In certain embodiments,electrical conductivity is assured by using an electrically conductivesolvent, for example by including at least one salt in the solvent toform an electrolyte. Certain embodiments have the electrode contact soilthrough an intermediate paper (such as filter paper or chemwipematerial) that has been wetted with a solvent, optionally anelectrolyte. In other embodiments, the contact is direct with the soilin intimate contact with the electrode.

The electrode functions without the need to exclude or remove ambientoxygen. Furthermore the analysis can be done without a need to addstrong acid (such as HNO₃, HCl, or H₂SO₄) and optionally can be donecompletely without pH adjustment, nor to acidify the soil or testingenvironment.

The solvent can be water and/or another solvent. In embodiments, thesolvent comprises a deep eutectic solvent, for example a deep eutecticmixture of ethylene glycol and choline chloride such as can be obtainedfrom a 2:1 mol ratio of ethylene glycol and choline chloride. Othersuitable solvents include room temperature ionic liquids (RTILs, whichare salts that are liquid at room temperature, not to be confused withsalts dissolved in another liquid), and high-boiling (thus having a lowevaporation rate) polar aprotic or protic solvents such as dimethylsulfoxide (DMS) or dimethylformamide (DMF). A low rate of evaporation isdesirable as it allows time for the analysis to be performed withoutrequiring a sealed chamber or the like to reduce evaporation.Preferably, the solvent(s) are able to dissolve and extract chloratefrom soils across a broad temperature range without significantevaporation. In embodiments, the solvent includes at least one salt (notnecessarily an RTIL) in order to ensure conductivity.

Embodiments can include conventional computer hardware (e.g. amicroprocessor, memory, etc.) and software sufficient to analyze theresults of voltammetry performed by a potentiostat and to determine alikelihood of the presence or absence of chlorate in a sample and/or theexpected concentration of chlorate in the sample (for example, bycomparing the shape of a voltage/current curve received from apotentiostat to stored examples representing curves obtained from knownchlorate concentrations), and optionally to transmit those results.Further embodiments can entail wireless transmission of potentiostatdata to a separate device (for example a mobile device such as acellphone or tablet) which is equipped with software and hardwaresufficient to analyze the data and provide a reading of chlorate level.Such embodiments might reduce the circuit complexity, power requirement,and weight of the remote system.

A system for conducting the technique can include a potentiostatoperably connected to an electrode comprising layers ofvanadium-substituted phosphomolybdate alternating with layers ofpara-rosaniline, and computer hardware and software in communicationwith the potentiostat and configured to produce an output indicating achlorate level in soil in contact with the electrode. Optionally, thesystem includes a reservoir of solvent and a valve configured to allowthe solvent to wet the soil, electrode, and/or paper, therebyfacilitating analysis.

In further embodiments, the electrode and potentiostat are carried by anunmanned vehicle such as an unmanned aerial vehicle, optionally with aradio transmitter configured to transmit the results of the analysis, orin the alternative configured to transmit potentiostat data as describedabove.

CONCLUDING REMARKS

A multilayer film, prepared via layer-by-layer deposition of cationicpara-rosaniline acetate dye (i.e., PR) and the vanadium-containingKeggin-type [PMo₁₁VO₄₀]⁵⁻ (i.e., PVMo₁₁) polyoxometalate anion, onindium tin oxide (ITO) provides an electrode for detection of chlorate.Taguchi L16 array and two-level full factorial statistically designedexperiments were used to probe the current response as a function of thecomposition of the electrode and analyte solution. Performance wasinvestigated as functions of the number of PVMo₁₁/PR electrode bilayers(L; 3-6 bilayers), solution acidity/pH (H; pH ˜1.32-2.85), solution[ClO₃ ⁻] (C; 250-1000 μM), voltage scan rate (S; 50-200 mV·s⁻¹), andfilm age (A; 1-8 weeks).

The Taguchi L16 array results indicated that maximum current responsewas obtained using 1 week old electrodes comprising 5 PVMo₁₁/PR bilayersscanned at 200 mV·s⁻¹ in pH 2.85 solutions containing 1000 μM ClO₃ ⁻.However, a significant film aging effect was also observed, consistentwith relaxation of film components kinetically trapped during initialdeposition to their equilibrium thermodynamic conformations with time, aprocess requiring at least 5 weeks at room temperature. A subsequenttwo-level full factorial design investigation of the effects due to theL, H, C, and S variables using 8 week old films identified the L, S, H,and C factors and HS and LS factor interactions as making statisticallysignificant contributions to the observed current. A model describingthe dependence of electrode current on the levels of these parameterswas derived, which included and confirmed the linear dependence on [ClO₃⁻] noted in early experiments.

A key finding of this work was the insensitivity of the film/electrodeto oxygen and common explosives such as TNT, which typically exhibitelectrochemical signatures in the same region as the chlorate reductionand the polyoxometalate component of the films. In fact, lineardependence of current on chlorate concentration over a 0 μM≤[ClO₃⁻]≤1000 μM range in aerated 0.10 M sodium acetate pH 2.5 (aq) solutionwas demonstrated with a detection limit of ˜220 μM ClO₃ ⁻ (S/N>3). Theability to determine chlorate under ambient conditions in this mannerand the insensitivity of the electrode to the presence of common N-basedexplosives bodes well for its eventual use in the field as a new tool toassist forces in the identification of IED manufacturing sites utilizingchlorate-based explosives. Work is currently in progress to understandand quantitatively map the effects of other environmental factors, suchas temperature and humidity, on electrode preparation, storage, andperformance and integrate the electrode system for deployment onunmanned aerial vehicle (UAV) platforms.

The major advantages of the described electrode are: (1) it is portable,small, and lightweight (as is the potentiostat/electronics needed tooperate it); (2) the electrode operates without interference by oxygen,so it can be used in the field (unlike other chlorate sensing electrodesystems, which required degassed solutions to function); (3) theelectrode does not respond to the common explosives TNT and RDX, whichwould be expected to also occur in the environment for which use of theelectrode is intended; (4) the electrode is rugged in that the compositefilm of para-rosaniline and vanadium-substituted phosphomolybdateadheres well to the indium tin oxide or glassy carbon electrode basesubstrate support; (5) the described electrode is easy to prepare via asimple dipcoating procedure that produces uniform and reproducible filmsof active para-rosaniline/vanadium-substituted phosphomolybdatecomposite; (6) the described para-rosaniline/vanadium-substitutedphosphomolybdate composite film is stable in that it retains itsactivity for catalyzed electroreduction of chlorate even after 8 weeksof dry storage at room temperature.

All documents mentioned herein are hereby incorporated by reference forthe purpose of disclosing and describing the particular materials andmethodologies for which the document was cited.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without departing from the spiritand scope of the invention. Terminology used herein should not beconstrued as being “means-plus-function” language unless the term“means” is expressly used in association therewith.

REFERENCES AND END NOTES

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What is claimed is:
 1. A system for analyzing chlorate, comprising: apotentiostat operably connected to an electrode comprising layers ofvanadium-substituted phosphomolybdate alternating with layers ofpara-rosaniline, and computer hardware and software in communicationwith the potentiostat and configured to produce an output indicating achlorate level in soil in contact with the electrode, wherein the systemis configured for performing voltammetry with the electrode so that acatalytic reduction current relates to the chlorate level.
 2. The systemof claim 1, further comprising a reservoir of solvent and a valveconfigured to release the solvent.
 3. The system of claim 1, furthercomprising a radio transmitter configured to wirelessly transmit datafrom the potentiostat and a radio receiver configured to receive thedata, wherein said computer hardware and software are in operableconnection with the receiver.
 4. The system of claim 1, furthercomprising a radio transmitter configured to wirelessly transmit saidoutput.